Ban called on all Iraqi authorities to ensure that the attack is properly investigated and its perpetrators held accountable.

"Attacks on places of worship are completely unacceptable and are prohibited under the international law," Ban said in a statement from his spokesperson.

Media reports put the toll in the attack at between 68 and 73. Ban expressed deep concern about the impact of sectarian violence on the already grave security situation in the country.

He also voiced his concern about the impact of such attacks on the political process, which aims to establish a unified government capable of confronting the threat posed by the Islamic State (IS) Sunni extremist group.

He urged all political leaders to unite and call on their constituents to refrain from sectarianism and to give the political process a chance, stressing that the sectarian attacks and counter-attacks only serve the enemies of Iraq.